A hard disk drive (HDD) is an essential information storage apparatus in computers and various consumer electronics products, particularly, for large capacity information storage application. The magnetic recording method is basically classified into two types of technical methods, one being longitudinal magnetic recording (LMR) and the other being perpendicular magnetic recording (PMR), depending on the direction of the magnetization vector in the magnetic recording layer of a magnetic recording medium. In recent years, it has been found that the longitudinal magnetic recording method has the recording density limit at about 100 Gb/in2 and the longitudinal magnetic recording method has being shifted to the perpendicular magnetic recording method in a magnetic recording hard disk drive. The advantage of the perpendicular magnetic recording method over the longitudinal magnetic recording method has been demonstrated by the attainment of the recording surface density above 300 Gb/in2.
Japanese Patent Publication No. 2006-48900 (“Patent Document I”) discloses a perpendicular magnetic recording medium having a first magnetic recording layer and a second magnetic recording layer ferromagnetically coupled to each other while sandwiching a coupling layer 6 therebetween. The coupling layer has any one of elements V, Cr, Fe, Co, Ni, Cu, Nb, Mo, Ru, Rh, Ta, W, Re, and Ir as a main ingredient and has a film thickness of preferably 2 nm or less. Patent Document 1 further describes that the ferromagnetic materials, Fe, Co, Ni can also obtain a coupling energy more suitable to the adjustment of alloying with a non-magnetic material, deposition condition, or deposition atmosphere. Further, it is described that the ferromagnetic coupling can also be obtained in the case of using Pd or Pt, but the anisotropic energy increases at the boundary between the coupling layer and the magnetic recording layer in the case of using Pd or Pt which is not suitable since this results increase the switching field.
According to the inventors, it is necessary that the perpendicular recording medium for a writing head with a shield on the rear end (trailing shield (TS) type head) has a relatively low medium saturation field (Hs) in order to maintain an appropriate overwrite (OW) level. According to the Stoner-Wholfarth (S-W) coherent magnetization rotational reversal model, the reversal field of the medium is inherently determined by the magnetic anisotropy energy density Ku. As Ku of the magnetic grain is decreased, the switching field is decreased. However, the medium thermal stability factor KuV/KBT (where V represents a thermal activation volume, KB represents a Boltzman constant, and T represents a temperature) is also a value which is determined being inherently related to Ku, and it is necessary that KuV/KBT is at least 60 in order to keep the recorded magnetic bit stably for 10 years. Thus, a question arises on how to obtain good thermal stability and medium writing performance simultaneously.
In the case of using an exchange-sprint medium where a soft magnetic layer and a hard magnetic layer are exchange-coupled to each other much improved writing performance can be obtained. In such an exchange-spring medium, the coupling layer plays an important role to enhance the exchange-spring effect. In the case where the vertical coupling is too weak, since the hard magnetic layer and the soft magnetic layer rotate separately, the magnetic reversal of the hard magnetic layer is not supported sufficiently by the soft magnetic layer.
None of the above-mentioned documents define the range of the saturation magnetization for the material of the coupling layer. Further, the optimal value for the thickness of the coupling layer is less than 1 nm in each of the examples. Since the recording properties of the medium utilizing the exchange-spring effect vary greatly depending on the thickness of the coupling layer, stable recording properties can scarcely be maintained in the case of mass producing such exchange-spring media.